This invention generally relates to self-checking flow meters based on the principles of ultrasonic transit time measurement. More specifically, the invention relates to a self-checking, transit time ultrasonic flowmeter which uses two or more chordal integration methods within a single meter body.
Because transit time ultrasonic flowmeters are capable of high accuracy performance over a wide range of application conditions, the meters have been adopted in applications ranging from custody transfer of hydrocarbons to measurement of nuclear feed water flows.
To achieve their high accuracy, transit time ultrasonic flowmeters commonly employ multiple pairs of transducers to infer velocity on a number of discrete chordal planes. The velocity measurements can then be combined, along with information on geometry, to produce a measure of volumetric flowrate, Q.
For the purpose of this disclosure, a path is an intended route of ultrasound transmission through the fluid between two transducers. A chordal path (or chord) is a path confined to a single chordal plane. A chordal plane is a plane that intersects two points on the boundary of a conduit and extends in a direction that is parallel with the central axis of the conduit.
Because velocity is continuous but can only be sampled on a number of discrete paths though the conduit, the meters can be prone to integration error. Because of this error, the measured flowrate derived from the velocity on multiple paths is not equal to the true flowrate. Even if the individual chordal path velocity measurements made by the meter have no intrinsic errors, identifying or quantifying the integration errors can be quite challenging. One way of identifying the integration error is to compare, using chordal integration methods, flow rate measurement results from different chordal integration methods in a single meter.
Chordal integration methods have been used in transit time ultrasonic flowmeters since the late 1960's. By choosing the path locations and combining the individual velocity measurements linearly according to rules for numerical integration, the result can represent the velocity integrated or averaged over the cross-section, and hence the volumetric flowrate, i.e.
            v      _        =                  ∑                  i          =          1                N            ⁢                          ⁢                        w          i                ⁢                  v          ⁡                      (                          h              i                        )                                    Q    =                            v          _                ⁢        A            =              A        ⁢                                            ∑              N                                      i              =              1                                ⁢                                    w              i                        ⁢                          v              ⁡                              (                                  h                  i                                )                                                        where Q is volumetric flowrate, v is average velocity, A is cross-sectional area, v(hi) is the path velocity at distance hi, and wi is the factor used to weight the velocity measurement before summation as illustrated in FIG. 1. Chord locations (hi) and weighting factors (wi) based on the rules of Gaussian integration are commonly chosen, based on either Legendre or Jacobi polynomials. Alternative integration schemes such as Chebyshev or Lobatto methods can also be applied.
More recently ultrasonic flow measurement methods have been developed which make use of two independent measurement systems in order to attempt to identify measurement errors. Examples include the combination of a 4-path chordal measurement with a single-path chordal measurement (see e.g. FIG. 3) and the combination of two 4-path chordal measurements (see e.g. FIG. 4). Although these new methods are generally better at detecting measurement errors than their predecessor methods, they are prone to other operating problems and are deficient when it comes to detection of integration errors.
In the case of the first method, the single-path measurement is much more sensitive to velocity profile changes than the 4-path chordal measurement. Therefore, the meter has a tendency to trigger an alarm at a level where the 4-path chordal measurement is still accurate.
The second method is potentially subject to “common mode” errors. Because the two 4-path chordal measurement systems are similar, any error common to both systems can be equal and, therefore, go undetected. To avoid this problem it is desirable to use two or more dissimilar multi-path chordal integration methods and that, in turn, increases overall hardware requirements. For example, to compare a 3-chord integration with a 4-chord integration, the meter body is required to have measurement paths in 7 chordal planes in total.
A need exists for an transit time ultrasonic flow meter that can accurately and reliably detect integration errors, avoids the deficiencies of the prior art methods, and reduces the overall hardware requirements that the use of dissimilar chordal measurement systems presents.